In general, an electric automobile uses electric energy stored in a battery, for its energy source. Lithium-ion polymer batteries are widely used for such batteries of electric automobiles and are being actively researched.
Meanwhile, in the case of a gasoline automobile which uses fuel (specifically, gasoline) in order to drive its engine, there is no difficulty in measuring the quantity of fuel in the automobile. However, in the case of the electric automobile which uses a battery, it is difficult to measure the energy remaining in the battery. However, for a driver of the electric automobile, it is very important to have information about the quantity of energy currently remaining in the battery of the automobile and the distance which the automobile can drive further by the remaining energy in the battery.
That is, the electric automobile runs by using the energy charged in its battery, and it is thus very important to recognize the State Of Charge (hereinafter, referred to as “SOC”) of the battery, i.e. the remaining capacity of the battery. Therefore, various technologies have developed, in order to detect the SOC of the battery during the driving and inform the driver of information about the distance which the automobile can run by the SOC, etc.
Further, there have been various trials for optimally setting an initial SOC value of the battery before the driving of the automobile. According to a typical method, the initial SOC value is set based on an Open Circuit Voltage (hereinafter, referred to as “OCV”). This method is based on an assumption that the OCV does not depend on the environment and serves as an absolute reference value for the SOC.
However, various tests and treatises have verified that the OCV does not have a fixed value regardless of the environment but has a value changing according to temperature and aging. However, the conventional method for setting an initial SOC value of the battery does not take the OCV changing according to the temperature into consideration, and it is thus impossible to estimate an exact SOC of the battery by the conventional method.
In consideration of such a problem, the applicant has proposed a method for setting an initial SOC value by using a table including OCVs and SOCs at various temperatures (Korean Patent Application No. 2005-19487). However, the proposed method is problematic in that the OCV refers to the voltage of the battery at a stable unloaded state and is thus unavailable any more after the battery is loaded. That is to say, chemical reaction occurs in the battery when the battery is in a loaded state, and the battery cannot reach the OCV state directly after an applied voltage is interrupted, but can reach a convergence state for the OCV after passage of sufficient time from the interruption of the voltage.
In other words, even when the applied load is eliminated, that is, even when the current flow is interrupted, the voltage converges to the a stable unloaded voltage or the OCV after passage of predetermined time, usually more than one hour. Therefore, if the SOC is initialized by the methods described above, hybrid electric automobiles may have the following problem.
When a driver restarts an engine of a hybrid electric automobile after passage of sufficient time after stopping the engine, it is possible to initialize the SOC in the method as described above. However, when the driver restarts the engine before passage of sufficient time after stopping the engine, the SOC is initialized by considering a voltage before reaching the OCV as the OCV, which results in occurrence of error.
In conclusion, the prior arts fail to consider environment of a device to which a battery is installed, specifically external temperature change. Further, even the prior art considering the external temperature change is problematic in that it initializes the SOC based on the voltage before the battery reaches the OCV.